Increase of carbon dioxide in the environment has been associated with the onset of global warming, the greenhouse effect. Portland cement manufacture is a very large contributor to the carbon dioxide emissions with approximately 0.9 ton of carbon dioxide emitted for every ton of cement made. Concrete is second only to water as the most consumed substance in the world (hundreds of millions of tons worldwide) and Portland cement, with its huge carbon footprint, is the principal ingredient in concrete.
The chemical process of making Portland cement is the reaction of limestone (calcium carbonate) with clay (hydrated alumino-silicate) at high temperatures. This elevated chemical reaction, called calcination, releases carbon dioxide to the atmosphere at a very high rate, for example, about 60% of the emissions from a cement plant. The high temperatures used for calcination reactions require combustion of carbon based fuels and are responsible for about 40% of the emissions of the cement plant.
The above mentioned factors associated with Portland cement manufacture cannot be replaced so there is little that can be done to reduce carbon dioxide emissions from a cement plant. Thus, there is great need for an alternative to Portland cement based concrete as a structural building material.
Numerous approaches have been tried with various degrees of success. Fly ash, a by-product of coal burning power plants is produced worldwide in large quantities annually; e.g., hundreds of millions of tons. Fly ash can be added to concrete mixtures but only about 10% of the fly ash produced annually is used in concrete for various reasons. A critical drawback of the use of fly ash in concrete is that initially the fly ash significantly reduces the compressive strength of the concrete as discussed by Ravindrarajah and Tam in (1989). Fly ashes from different sources may have differing effects on concrete. Fly ash may behave differently depending on the type of Portland cement used (types I-IV) since they have different chemical compositions. (Popovics, 1982).
Liskowitz et al. in U.S. Pat. No. 6,802,898 B1, (2004) describes a method for preparing fly ash for high compressive strength concrete and mortar and shows that it is possible to increase the strength of concrete containing fly ash by grinding the fly ash to a desired size distribution and increasing the yield of fly ash that can be used in a specific mixture of concrete. However, the percentage of fly ash that can be used in a concrete mix with Portland cement even with grinding to a specific size distribution of particles is limited to 10-50%. The costs and maintenance of agglomerated free fly ash powders limits the use of this process. The concrete industry typically limits fly ash to less than 30% in concrete mixtures, thus, only a small fraction of concrete contains any fly ash.
Another alternative to reduce the use of Portland cement based concrete is to use a process termed geo-polymerization to manufacture structural building materials. These materials, called geo-polymers, are synthetic analogues of natural zeolitic materials, as reported by Davidovits, et al. in U.S. Pat. No. 5,342,595 (1994) and van Jaarsveld et al, in “The Effect of Composition and Temperature on the Properties of Fly Ash and Kaolinite-based Geopolymenrs, Chemical Engineering Journal, 89 (1-3), pages 63-73 (2002).
Geo-polymers are created by chemically dissolving silicon and aluminum-containing source materials at high pH in the presence of soluble alkali metal silicates. The three principal process steps are: 1) dissolution of the aluminum and silica containing raw materials to form mobile precursors through the complexing action of hydroxide ions, 2) partial orientation of mobile precursors as well a partial internal restructuring of the alkali polysilicates, 3) re-precipitation where the whole liquid system hardens to form an inorganic polymeric structure that can be amorphous or semi-crystalline.
In order to form the geo-polymerized structure, it is essential to dissolve completely the silicon and aluminum containing source materials according to van Jaarsveld, et al, 2002 supra. Geo-polymers do not utilize the formation of calcium-silica-hydrates for matrix formation and strength but instead depend on the polycondensation of solubilized silica and alumina precursors and high alkali content to attain structural strength.
Typical formulations of geo-polymers involve dissolution of fly ash and calcined kaolinite with various quantities of sodium or potassium silicate and sodium or potassium hydroxide. Strength of the resultant geo-polymer depends greatly upon fly ash/kaolinite ratio and calcination temperature (300-900 C) of the aluminum-silicate containing precursor (kaolinite) as reported by van Jaarsveld, et al, 2002 supra. It is possible to vary many geo-polymer process characteristics, such as ratios of clay to fly ash, calcination temperature of the clay, water/fly ash ratios, etc. However, the strength of such geo-polymer materials seldom is equivalent to Portland cement based structural materials; geo-polymer strengths are in the range of 5 to 11 MPa (725-1500 Psi) whereas Portland cement based concrete must be in the range of 20-40 MPa (3,000 to 6,000 Psi). Further discussion of geopolymers used or designed for structural materials is by Jaarsveld et al, in “The Effect of Alkali Metal Activator on the Properties of Fly-Ash Based Geopolymers,” Ind. Eng. Chem. Res, 38 (10) (1999) 3932-3941; Madani A et al, “Si-29 and Al-27 NMR-Study of Zeolite Formation from Alkali-Leached Kaolinites—Influence of Thermal Preactivation,” Journal of Physical Chemistry 94 (2):760-765 (1990); H. Rahier et al, “Low-Temperature Synthesized Aluminosilicate Glasses” Chapter 3. Influence of the Composition of the Silicate Solution on Production, Structure and Properties, Journal of Materials Science 32 (9): 2237-2247 (1997); J. Davidovits, “Synthesis of New High Temperature Geopolymers for Reinforced Plastics/Composites,” Proceedings of PACTEC 79, Society of Plastic Engineers, 151-174 (1979); and J. Davidovits, “Process for the Fabrication of Sintered Panels and Panels Resulting from the Application of this Process,” U.S. Pat. No. 3,950,470 (1976).
A related technology has been described by Nilsen et al in “Preparation and Characterization of Binder for Inorganic Composites made from Amorphous Mineral Raw Material, Journal of Sol-Gel Science and Technology, 35 (2), 143-150 (2005) for making an inorganic composite binder material through a sol-gel route using alumino-silicate amorphous mineral raw materials containing alkaline earth and transition metal oxides. The Nilsen et al method requires complete dissolution of the starting raw material in formic acid. The strengths that are developed are not sufficient for use in structural applications.
Another route used in chemical processing to produce structural materials that possess properties equivalent to Portland cement-based concrete without characteristic carbon dioxide emissions is to use sol-gel processing. Hench et al, in U.S. Pat. No. 5,147,829 disclose sol-gel derived SiO2 oxide powder composites and their production and discuss how to incorporate small oxide powders having a diameter size range between approximately 0.001 to approximately 10 microns in a silica based sol to form a composite material. The composite produced thereby was a monolithic silica gel matrix with homogeneously distributed oxide powders with mechanical properties equivalent or superior to Portland cement based concrete.
The curing time of the silica sol-based composite of Hench et al was substantially more rapid than Portland-cement based concrete. However, the percentage of oxide powders contained within the silica sol based composite is limited to 1% to 10% by weight, the remainder being silica gel which requires heating the composite to elevated temperatures greater than 700° C. for drying and stabilization. The low concentration of oxide powders in the silica sol and high temperatures required for stabilization and densification and cost of the silica alkoxide precursors do not make this type of process economically suitable for replacement of Portland cement based structural materials.
More technological innovation is needed to provide stronger, lighter, cheaper, and more reliable structural materials that can replace and surpass the existing use and reliance on Portland cement and the manufacture thereof to significantly reduce the carbon footprint of the manufacture of cement-based structural materials.